Transmission systems known from practice and designed as automatic transmissions are designed to obtain various gear ratios by means of shifting elements which can in each case transmit an applied torque by friction. When there is a command to obtain a defined gear ratio, in each case at least one or more of the frictional shifting elements are disengaged from the force flow of a transmission while at least one or more other frictional shifting elements are engaged in the force flow of the transmission to obtain the gear ratio required. During the process of engaging a frictional shifting element, no special synchronization measures are needed to ensure a desired shifting comfort since the shifting comfort desired can be achieved with frictional shifting elements at defined contact pressures within a broad range of rotational speed differences.
Since, in a manner known per se, such transmission systems can only be operated with insufficient efficiencies because of drag losses that occur in the area of open frictional shifting elements, certain frictional shifting elements are replaced by interlocking shifting elements. In such automatic transmissions, made with both frictional shifting elements and with at least one interlocking shifting element and designed with a corresponding gearset structure, to carry out a defined shift command at least one interlocking shifting element has to be engaged in a force flow of the transmission.
Compared with frictional shifting elements, interlocking shifting elements with or without additionally designed synchronizing devices can only be engaged comfortably when the speed differences are very small, i.e. close to the synchronous speed, and for that reason the operation of a transmission made with at least one interlocking shifting element and designed as an automatic transmission is made more difficult to an undesired extent, at least while carrying out certain shifts that involve an interlocking shifting element.
Moreover, if shifts are to be carried out without traction force interruption, then for example in planetary transmissions interlocking shifting elements or claw-type shifting elements can only be used if they are disengaged during upshifts from a lower gear to a higher gear. In turn, a result of this is that interlocking or claw shifting elements are essentially engaged in a force flow of a transmission to obtain low gears, during which a drive machine of a vehicle is temporarily stopped depending on the operating condition if the motor start-stop function is activated.
Interlocking shifting elements are engaged by means of electric actuators preferably designed as pilot valves. Among these a distinction is made between pilot valves in which the pilot pressure, as a function of which the actuating pressure of the interlocking shifting element is determined, increases as the control current decreases, and those in which the pilot pressure decreases as the control current increases.
If an interlocking shifting element is actively engaged by means of a pilot valve in which the pilot pressure is proportional to the control current, then when the pilot pressure is reduced the interlocking shifting element is disengaged if the actuating pressure is likewise proportional to the pilot pressure. By controlling the interlocking shifting element by means of a proportional pilot pressure valve, to ensure a desired safety standard in the event of a failure of the electrical control system, the interlocking shifting element should be moved to a safe condition, preferably its open operating condition.
Since hydraulically actuated shifting elements are usually supplied with hydraulic pressure by means of a hydraulic pump driven by a drive machine, when the drive machine stops preferably by virtue of a motor start-stop system the hydraulic supply pressure in the hydraulic system of a transmission falls. The claw shifting elements engaged in low gears, which for example are designed with a simply acting piston-cylinder system and with a spring device that acts in the opening direction of the interlocking shifting element, are changed by the restoring spring to an open operating condition when the pressure supply is cut off.
However, interlocking shifting elements of such a design have the disadvantage that during driving operation the restoring spring has to be constantly counteracted if the interlocking shifting element is to be kept in its closed operating condition. Here, it is particularly problematic that a high restoring force has to be provided by the restoring spring, since it must be possible to disengage the interlocking shifting element dynamically and quickly in opposition to internal friction forces and a hydraulic resistance of the hydraulic system.
To avoid spontaneous disengagement of an interlocking shifting element, owing to the high restoring force in the area of the restoring spring, a high actuating pressure acting in the closing direction of the interlocking shifting element must be provided.
On the other hand, a lasting engagement of a claw shifting element by means of an engaging spring that acts in the closing direction of the claw shifting element should be avoided for safety reasons, since if the disengagement pressure of the claw shifting element is not controlled correctly the interlocking shifting element would be automatically engaged.